Barrier properties of model coatings using recycled iron oxide pigment

Abstract: The aim of this study was to compare the barrier properties of organic coatings with the same polymer matrix but different types of iron oxide particles, commercial and recycled. The work was carried out for the company SSAB with the aim of investigating whether the pigment Merox ferric oxide, which is produced in-house at the company during the pickling process, can be used and replace commercial iron oxide pigment. Evaluation of the barrier properties was done using electrochemical impedance spectroscopy where the capacitance of the coatings is calculated and reflects the water absorption of the coating over a period of exposure to the electrolyte. By comparing the capacitance of the different coatings, an estimation of the water transport through the different coatings was made. Complementary methods used were digital optical microscopy and scanning electron microscopy.  In this project, 6 lab samples were evaluated. In common, the samples consisted of the same metal substrate, hot-dip galvanized steel, and the binder in the coatings was polyester-melamine. Sample 1 consisted of the binder only, sample 2 contained the commercial iron oxide pigment, samples 3 and 4 consisted of Merox pigments but different levels of chlorine in the pigment. The thickness of the coatings of the 4 samples was 6 m. Sample 5 had a thicker coating of 20 m and contained Merox pigment. The last sample evaluated was a reference sample with anti-corrosive pigment with a thickness of 20 m. Furthermore, exposure tests were carried out with sample 5 to test how different electrolyte solutions affected the samples. The electrolytes tested were 1 M Na2SO4 (for reference), 1 M NaNO3 and 0.1 M NaCl.  The impedance measurements were performed in 3 parallel measurements on the samples with thinner coating (6 m) and an average of the capacitance was used to analyze how the capacitance changes over time. The results showed that sample 1 had much better barrier properties than the samples containing iron oxide pigments, although the exposure time to electrolyte was longer for this sample, the capacitance was lower. Both samples 2 and 4 increased rapidly in capacitance over a short period of time. Sample 3 had a rapid increase at the beginning which later decreased, the sample showed better barrier properties compared to samples 2 and 4. Cross-sectional studies carried out by scanning electron microscopy detected corrosion formation between the zinc and the coating on all samples with thinner coatings. Only one impedance measurement was made on samples 5 and 6, hence multiple measurements need to be performed to calculate the capacitance over time and provide complete and reliable results for these samples.  The results of the exposure tests indicated that the electrolyte used for the electrochemical impedance measurements, 1 M Na2SO4 is more corrosive than the other electrolytes tested, 1 M NaNO3 and 0.1 M NaCl.